J/A+A/555/A109 14N/15N isotopic ratio in L1544 (Bizzocchi+, 2013)
Detection of 15NNH+ in L1544: non-LTE modelling of dyazenilium hyperfine
line emission and accurate 14N/15N values.
Bizzocchi L., Caselli P., Leonardo E., Dore L.
<Astron. Astrophys. 555, A109 (2013)>
=2013A&A...555A.109B 2013A&A...555A.109B
ADC_Keywords: Atomic physics ; Molecular clouds ; Radio lines
Keywords: ISM: clouds - molecules - individual object (L1544) -
radio lines: ISMC
Abstract:
Samples of pristine solar system material found in meteorites and
interplanetary dust particles are highly enriched in 15N.
Conspicuous nitrogen isotopic anomalies have also been measured in
comets, and the 14N/15N abundance ratio of the Earth is itself
higher than the recognised presolar value by almost a factor of two.
Ion/molecules, low-temperature chemical reactions in the proto-solar
nebula have been repeatedly indicated as being responsible for these
15N-enhancements.
We have searched for 15N variants of the N2H+ ion in L1544, a
prototypical starless cloud core that is one of the best candidate
sources for detection owing to its low central core temperature and
high CO depletion. The goal is to evaluate accurate and reliable
14N/15N ratio values for this species in the interstellar gas.
A deep integration of the 15NNH+(1-0) line at 90.4GHz was obtained
with the IRAM 30m telescope. Non-LTE radiative transfer modelling was
performed on the J=1-0 emissions of the parent and 15N-containing
dyazenilium ions, using a Bonnor-Ebert sphere as a model for the
source.
A high-quality fit of the N2H+(1-0) hyperfine spectrum has allowed
us to derive a revised value of the N2H+ column density in L1544.
Analysis of the observed N15NH+ and 15NNH+ spectra yielded an
abundance ratio N(N15NH+)/N(15NNH+)=1.1±0.3. The obtained
14N/15N isotopic ratio is ∼1000±200, suggestive of a sizeable
15N depletion in this molecular ion. Such a result is not consistent
with the prediction of the current nitrogen chemical models.
Since chemical models predict high 15N fractionation of N2H+, we
suggest that 15N14N, or 15N in some other molecular form, is
preferentially depleted onto dust grains.
Description:
The observations towards L1544 were carried out with the IRAM 30m
antenna, located at Pico Veleta (Spain) during observing sessions in
June 2009 and July 2010. The J=1-0 transition of was observed with the
EMIR receiver in the E090 configuration tuned at 90263.8360MHz and
using the lower-inner sideband. The hyperfine-free rest frequencies
were taken from the most recent laboratory investigation of
15N-dyazenilium species (Dore et al. 2009A&A...496..275D 2009A&A...496..275D).
Objects:
----------------------------------------
RA (2000) DE Designation(s)
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05 04 18 +25 11.1 L1544 = LDN 1544
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tableb1.dat 101 1537 N2H+/H2 hyperfine de-excitation collisional
rates
tableb2.dat 101 190 N15NH+/H2 hyperfine de-excitation
collisional rates
tableb3.dat 101 190 15NNH+/H2 hyperfine de-excitation
collisional rates
tableb4.dat 37 55 N2H+ hyperfine level energies
tableb5.dat 32 19 N15NH+ hyperfine level energies
tableb6.dat 32 19 15NNH+ hyperfine level energies
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See also:
J/ApJ/664/928 : Starless cores in HCN(J=1-0) (Sohn+ 2007)
J/ApJS/152/81 : CO survey toward starless cores (Park+, 2004)
J/ApJ/526/788 : Survey of infall motions toward starless cores (Lee+, 1999)
J/ApJS/125/161 : A database of dense cores mapped in ammonia (Jijina+, 1999)
Byte-by-byte Description of file(#): tableb1.dat tableb2.dat tableb3.dat
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Bytes Format Units Label Explanations
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1- 4 I4 --- Row Row number
8- 9 I2 --- i [1/55] Initial level (1)
12- 13 I2 --- f [1/55] Final level (1)
16- 24 E9.4 cm+3/s Rate5 De-excitation rate for T=5K
27- 35 E9.4 cm+3/s Rate10 De-excitation rate for T=10K
38- 46 E9.4 cm+3/s Rate51 De-excitation rate for T=15K
49- 57 E9.4 cm+3/s Rate20 De-excitation rate for T=20K
60- 68 E9.4 cm+3/s Rate25 De-excitation rate for T=25K
71- 79 E9.4 cm+3/s Rate30 De-excitation rate for T=30K
82- 90 E9.4 cm+3/s Rate40 De-excitation rate for T=40K
93-101 E9.4 cm+3/s Rate50 De-excitation rate for T=50K
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Note (1): the energy is given in column#1 'n' of tableb4/tableb5/tableb6
for for tableb1/tableb2/tableb3
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Byte-by-byte Description of file(#): tableb4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
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3- 4 I2 --- n [1/55] Energy level index
9- 20 F12.4 MHz Freq Level energy
27 I1 --- J [0/6] Rotational quantum number
32 I1 --- F1 [0/7] J + I1 quantum number
37 I1 --- F [0/8] F1+ I2 total angular quantum number
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Byte-by-byte Description of file(#): tableb5.dat tableb6.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
3- 4 I2 --- n [1/19] energy level index
9- 20 F12.4 MHz Freq level energy
27 I1 --- J [0/6] rotational quantum number
32 I1 --- F [0/7] J + I quantum number
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Acknowledgements:
Luca Bizzocchi, bizzocchi(at)oal.ul.pt
(End) Patricia Vannier [CDS] 10-Jun-2013